Pipe Design

Hi, Nobby1.
If i read you correctly , you are asking if there is any establishable ratio between cylinder volume and exhaust system volume !
Here are a few examples i can use as illustration .
RSHonda - - 32
TZYamaha- - 37
Malossi 50- - 48 . The 50 is interesting ,it`s a 2011 developement , bangs out 21 bhp and bmep of 13 bar , impressive !
I tried to find details for a 250 single ,like the Ossa and a 500 single like the Qub/Seeley , and whilst i have photos , i have no
dimentions ,so a dead end there.
Pipe volume is a function of length and diameter, length is a function of exhaust timing, temperature and gas composition.
Diameters , in the main, are dependent on the wave energy available and crankcase pumping pressure . There is no point in
asking the diffuser to suck on an empty crankcase , delivery ratio is crucial here, and, quite low in a Bantam .
With all of the above factors , and there are many more to include , the variables are so many that establishing a workable link
is nigh on impossible. I have yet to see ,in any published work , just such a ratio being established or even verifiably suggested .
Without reading the whole thread that you refer to, i coud not guess at a possible motive for suggesting a direct link .

Regards Trevor

thanks for reply..it was something ide seen quoted alongside an exhaust diagram and wonderd if it had any validity.. can you explain the low delivery ratio and why its so low in the bantam engine? also cant the waves produced in the conbustion / exhaust cycle be amplified or strenghtend to improve the return wave?

Hi Trevor
good question Nobby - over to you Trevor

Trevor it would be great if you shared with us the issues you are dealing and contending with on your current engine and return, I have shared what problems, I have had "usually to see what other have to say" but more importantly, how they resolved that particular issue, i feel it would be helpful to other builders and those nearing or about to start development and racing.

Nobby - read aprilia RSA ON - pitstop.biz page 23 onwards is very interesting and some very direct opinions about formulas and calculations for exhaust pipes, suggesting you could even start with a sq or round box on the end of a pipe, where you end up will be down to the experiments you can or are able to do, this guy even resorted to introducing nuclear cells and taking exrays during the ccycle while under load on the dyno, mind you coould not possible image the cost of doing this, but shows just what some factiories are prepared to do to find the correct/compromise to win races.

I was reading this, that I have commented on before, but I'm sure they said formula's, calculated systems no mater what you use are a waste of time, you can only find the correct dimesions/compromise (if there is such a thing) for power by experimenting on a dyno. ?

not copied and pasted this time Trevor, quoted Mr Jan and Mr Mars.

My opinion "for what it is worth", is - copied pipes give (better than calculated pipes) excellent starting points, as when I went to the efforts, of making one that was calculated from formula#s, "it gave a lot less power in fact 50% less on the dyno, than the one i got from ALAN's, i juts feel it all depends on too many variables, so pick one and experiment with it.

or one copied from an-others bike, is helpfull, as long as that bike works, is great and always helpfull, not sure every one wants you to measure there pipe in the paddock, but i have been offered the option from most of the front runners, always a good starting point,

Alan has a number he posts on here too, that have worked ok on some bikes.


regards Derek

trevor , have you ever used any of the ceramic coatings available to protect your engines from the sometimes high engine temps that the air cooled engines produce?

Nobby1,
Haven`t ignored you , will reply to all you questions shortly .

regards Trevor

thats ok... while im here had any use for fluid diodes to help with back flow of inlet charging..? read it can increase the charging phase on piston port engines at lower rpm speeds...??

Hi, Nobby1
Delivery Ratio,the mass of charge deivered to the cylinder during the scavenge period as compared to the mass
that could could fill the cylinder volume . So,all events that control the open cycle ,including pressure, temperature
and pipe gas dynamics relate to the magnitude of this number . A good RS125 is at around 1.4 , the Aprillia tops
1.55 and top of the heap F1 engines 1.75 . Accepting that average Bantams produce barely half the power of the RS
then it follows that the delivery ratio is correspondingly lower , depressing i know so we must all try harder !

Can energy be recovered or enhanced in a pipe, yes and no ? Sounds a bit of a copout ,but taking the Newtonian
law that energy cannot be created or distroyed , but can be mutually converted, will give a hint as to where we are
going with this ! The maximum pressure and temperature we can use to the engines benefit occur from exhaust
port opening to around 15 degrees after . Temperatures fall from perhaps 850c in the header to 100c at the tailpipe.
and with temp drop, the local speed of sound and gas velocity both diminish.
The bigger the cossectional area of the pipe and the entire surface area will determine the heat loss and pressure,
the more work that a rapidly expanding diffuser imposes on the available energy will detemine the returning
plugging pulse magnitude .If you are stuggling to make the diff work effectively, energy can be recovered by
increasing the cone angles but you always pay the price of reduced duration , depends what you want , a long
steady suck or a shorter more intense one . Too sharp an angle at the highest temp and wave speed could lead to
flow separation from the pipe wall ,and that really messes things up !

Ceramic coatings , no i have never tried or even contemplated their use , it has been used in Bantam exhaust
ducts, but as for any benefit i simply don`t know . That such coatings work is not in dispute but how to quantify
their effect and, therfore benefit for Bantams, i`m not sure ,perhaps some one else on the forum could help here,
might cost a bit too .

Fluid diodes work very well in reducing reverse flow in both piston port engine inlet ducts, and also in transfer ducts,
specific improvements in fuel consumption and smoothness of running are reported in low to medium speed engines.
It is strange to think that such a bit of kit with no moving parts could work so well, maybe something for one of you
enterprising young tuners to get stuck into ?

Hope it all makes sense, it`s at times like this that i do envy technical journalists that write on complex subjects
and explain it all so that dummies like me can get a handle on it all !


regards Trevor

thanks for the info trevor.

Ceramic coatings for the combustion chamber I have a wee bit of knowledge on
from working in R&D with Ford Dunton, KHD***Cologne and Deutz Diesel Canada Inc... The improved performance results were pleasing to see but in each case
the ceramics had no staying ability in whichever fashion they were used ...
One hundred to two hundred hours to failure in a 1,000hours durability test was not on for production...
So I would think in a resonant exhaust pipe with much expansion, contraction & vibration, a ceramic coating -- inside, of course -- would not last long -- yet maybe long enough for racing

-- but that´s a thought? has anybody considered lagging an exhaust system to keep heat in because with the air rushing past it is constantly cooled.

** Klöckner Humbolt Deutz Cologne.

Lagged our exhaust in about '84 if I remember correctly John for that very reason but made no perceptible difference apart from keeping my leg cooler !!

Bravo!
If that´s the case then the Ejector Jet principle would work here
where much like KERs and the Spitfire oil-cooler, ram-air rushing
into a duct (surrounding the exhaust from just before the diffuser
to just past the tail-pipe) meets a heated restriction which
causes the ram air to rapidly expand in this further confined space
and then speeds up the air to produce a jet thrust at the exit of
the duct....
On Mike powell´s exhaust (all nicely in line) it would work a treat!

Ian suggested I take a look at this thread and although I don't normally spend time on forums I thought I would chip in with my twopenn'orth on this thread. Hope somebody finds it useful.

There's lots of formulae available for exhaust pipes but I would recommend a look at the "Expansion chamber design utility" program freely available at www . iwt . com . au / pipedsn . HTM (without the spaces put in as I was not allowed to post!)

Its based on formulae developed by Gordon Blair, gives good results and saves you a lot of messing around with calculators or spreadsheets.

You need to know your exhaust port opening timing in degrees AFTER TDC and the area of the exhaust port. There is help in the program under "Tools" to find the area of your exhaust port or if that looks too tricky you can hold graph paper in the cylinder and rub the edge of the port with a pencil to show the outline on the paper. Count the squares (and half squares) to get the area. Divide the area by 0.7854 and take the square-root to get the "Effective diameter" and convert to mm if necessary.
There are ways to measure the area of an irregular shape with software but that would take another long message.
( If you need to know, 0.7854 is pi divided by 4 and saves you the trouble of multiplying by 2 after taking the sq. root )

1. The program invites you to choose a 2 or 3 stage diffuser - choose 3
2. The next choice is Exhaust Temperature choose "Calculate from Target BMEP"
3. Enter the Effective diameter, the diameter at the barrel flange and the length of the port from piston to flange.
4. Under "Diameter coefficients" select "Calculate from Target BMEP"
5. Enter Exhaust timing ATDC and the rpm you hope to obtain maximum power

When you chose the Three stage diffuser, another box popped up for the "Horn coefficient" which affects how rapidly the diameter expands towards the centre section - around 1.5 works well here.

When you chose "Calculate from target BMEP" anothe box popped up for your estimated BMEP.

Although Bantams rarely achieve a BMEP of 9 (bar) its fine to enter more than that in the BMEP box and 9.4 to 10 seems to work well.

Click on Calculate and you will get a set of dimensions that will work well. There's scope to play with the values given and you can work outwhat the bmep is for a given hp and engine size - also in the tools section.

If you have already tried it, have you any observations you can share ?

Mick

Now thats what i'm talking about, precise and something i can even understand.


Thanks Mick

Hi Mick Scutt

- just been trying out that software you suggested, I have played with it before, but never got any numbers out that equated to what I was expecting, and as I have already been well and truly put in my place with quotes about software, with others quite rightly pointing out, results are equal to the information entered i.e. garbage in = garbage squared out!!!, so holding my tail between my legg's for a moment, can you help me as I’m sure it must be to do with something I’m not doing correctly.

so can you help clarify a few things - questions, why does the centre section diameter come out so small, in most cases its around 72/77mm, no matter what I put in, the diameter is smaller than I’m expecting, -

while all the pipes I have ever made, are from the Gordon Jennings book formulas and they come out at least 100mm+.

I have also been lead to believe you have to go to 115/ 120mm to get a moden 2 stroke to breath correctly, and while I have blown pipes this big in the past, they just don’t work, but looking round the paddock some of the current front bikes pipes (125) are huge, except Trevor’s looks a lot smaller.

Not sure you seen the piece Trevor did on this, it was great, his came out at 108mm centre section, so by today’s standards, why does this software come out so small on the diameters, what am I doing wrong" !!!.

The software "is" very easy to use, and work with", also I went into the BMEP calculations software on the same page, entering my known values, bore stroke and power output, of the dyno - seems I only have 5.92bar BMEP. This was a little disappointing to me but im sure it may not be correct, seems you are quoting 9+bar, to make the formula work, but what it did tell me is - I need 23HP as a minimum? with a 54x54 engine to get 8bar, and the software not to throw out errors, so my other question is the same what am I doing wrong here. ( i used all the details you suggested)

my entries were as follows.
31.5 exhaust port window diameter /area?.
35 outlet diameter
86 deg atdc
1.5 and 8.8 from your suggestions
10800 rpm.

I would really like a response "anybody please feel free to comment too"

regards

Derek

Hi Derek

Its good to see you have tried it. The BMEP value of 8.8 is realistic but lower than suggested but otherwise I don't think you are doing anything wrong.

I tried your numbers and they do indeed indeed give small centre section diameters and although you will see modern two strokes with 120mm or larger diameter they are on engines that produce *much* higher BMEP and have exhaust ports that wrap nearly half way round the barrel!

see http://www.bonavolta.ch/hobby/en/moto/rs125r.htm

For a good Bantam I would expect a centre diameter between 98 and 108mm and for the record Ian's pipe this year has been 105mm diameter.

If you play around with the numbers you find the boxes that affect centre section diameter are the BMEP and the exhaust port effective diameter.

The BMEP is used to calculate the ratio of the various diameters based on Gordon Blairs experiments and you are at liberty to play with that value or even to choose the "Input Manually" option. If you try that I would suggest values around 3 and 0.7 but be careful to keep the tailpipe diameter no less than 22.

Even if you use higher BMEP values such as 10.5 with your other numbers the diameters come out on the small side - thats actually a useful bit of information and points to the effective exhaust port diameter as the culprit.

Trying 33 to 35mm effective diam. you start to get values you see in the paddock. The difficult bit is getting your exhaust port that big without destroying the piston ring. You can go for the Yamaha TZ style wide but well rounded type (see below) or a bridged exhaust that is wider and flatter at the top to give a good bang down the pipe as in the above link.

Both types need a bit of ingenuity to get them in a Bantam barrel but I'm told the Brian White alloy castings now have exhaust bridges.

see http://erlenbachracing.net/portmaps.htm for TZ dimensions

Hope that helps
Mick

Hi Mick,

With regards to the 1.5 & 9.4 to 10 firgures you mention are these adaptable to a 175?, also is there a safe standard of maximum rpm to go for on a 175 or is this all relative to your portings?



Regards


Eddie

many thanks Mick

I hope we will see you in the paddock again some time.

best regards Derek

Hi Ed

As a 175 has a heavier piston than a 125 the maximum revs need to be lower but this clearly depends on the components you use. If the conrod is from a high revving engine with a similar weight piston then clearly you *could* run at similar revs and be reliable but there are other considerations.

Its no surprise that a 175 needs more gas through the ports than a 125 so the ports need to be larger than in a 125. If there were no constraints on design then you would fill the bore with ports but that cannot be done on a 175 because space is limited by the position of the cylinder studs and the formula which forbids changes to the external shape of the crankcases.

The exhaust and inlet can be made larger without too much difficulty but but the main limitation is the transfer port area particularly at the crankcase and its this that puts a limit on how high you can push the power band on a 175. I'd suggest a good region to aim for is for max power around 9500 to 10000 assuming you have a suitable piston & con-rod.

Exhaust port width can go up to around 40mm (thats 40mm as a chord or 44mm as an arc-length) on a 62mm bore provided it is rounded at the sides to feed the piston ring back into its groove smoothly and you are using a fairly modern type of piston with a thin ring. For the inlet go as wide as you can with the lower piston skirt still supported at the sides by at least a couple of mm each side as it passes over the port.

Timings in the region of 97 bbdc for exhaust and 97 btdc for the inlet will pass enough gas for these rpm and the pipe program will give good results with a BMEP of around 10.5 or Manually input Diameter coefficients of 3 and 0.7 and a minimum tailpipe diameter of 23.5

If you follow these guidelines and get your exhaust port effective diameter up around 35mm it *will* be fast but don't get carried away with compression ratios. I know there are some using as much as 12:1 but with big ports and a good pipe I would suggest only 8.5 or 9:1 with ignition timing about 1mm btdc.

Mick

non technical.....HI Mick long time no see, hope your well mate.

Same for me Mr Scutt SIR! Welcome to the fold Mick...!
I am really glad Ian got you posting on here -- I was beginning to feel the heat because, as you know, I was not too bad with a welding torch but quite poor (mostly laziness & letting someone else do it) with engine tuning....
You can tell them how JayBee used to foam at the mouth at the Hoop & Grapes
over fairings, members not paying their subs and riders not paying up for late race entries etc... etc....

I bet you are proud of Ian´s success....!!

PS -- Re static (measured) compression ratio I agree (and so does Slick in the Island) with you. Slick reckons to go no higher than 8.5:1: measuring from the
top of the exhaust port for swept volume and then tuning the system to get a higher dynamic CR -- although I would add that with a methanol fuel the CR ought to be over 16:1....?????
Remembering what Trevor said that an RS125 has a `filling capacity´ of 1.4 times...
and Aprillia has 1.55 ... I assume it could be said that the dynamic compression-ratio is increased from say, a static 8.1, to 8 x 1.4 with the RS125 = 11.2 AND with the Aprillia, 8 x 1.55 = 12.4:1 ....

Is that a correct assumption?

With this in mind I might be able to excuse my unreliability of Icarus-2 where I had
unbeknowingly tuned its ports too well and ran on too weak a mixture...??


Cheers!

Hi Mick, John and all

I really dont want anybody getting confused on this - it could be expensive on pistons !

For a 175 engine on petrol I am suggesting a *geometric* compression ratio of 8.5 to 9:1 - thats about 23 to 22cc head volume at tdc. That may seem rather low but is about all a 175 cylinder can stand if its breathing well i.e. has big ports and a good pipe.

John, I expect your assumption is correct and I know you like working things out so here are some numbers I have to hand for a 25hp engine - 2 bar in the cylinder at 175 degrees C as the exhaust port closes. Its a 54 x 54 motor and the exhaust port height is 27mm

Mick

I shall have to resurrect my PV to the 1.35 or is it 1.41 index? -- I forget how to do it -- only joking ...??

Just like Titus Oates said during the fateful trip with Scott to the antartic when he walked out into the snow, "I am just going out -- and might be a little while..."

Whoops!

Started calculating and realised I had a problem....

With a diesel it is compression of pure air and with a two-stroke there´s petroil mixture of fuel-air-oil ...

Help!!


Trevor -- please -- what is the the ratio of constant-pressure & constant-volume
specific heats of a fuel-air-oil mixture?? Is it between 1.3... & 1.41 -- or is it larger because of being a mixture??
Pure air has a ratio of specfic heats of 1.41 when considering isentropic compression which alters with heat loss ( temperature rise and I imagine that the mixture of fuel AND oil has a changing ratio of specific heat values with changes of temperature???????
...
Or, would it make much difference to calculate as if just air??
...


Iza Hasbinn -- sulking....

Hi Mick,

Thank you for taking the time to explain all the parts out properly.


Kind Regards



Eddie

Hi john,
Without ploughing through my text books on thermodynamics and then through the Boyle and Charles laws , and, off the top of my head ,
the following might help.
PV=RT where R=the universal gas constant T=temp
You are quite correct in that the specific ratio of heats is 1.4 at 25*c . but is dependent on gas composition and in average race engines is
1.35 . It does vary throughout the whole engine cycle but 1.35 is very resonable average for twostroke fuel/oil mix .
The universal gas constant is also dependent on gas composition but primarily on temperature and is 287j/kg-k* for air at 25*c and is used at
290 for hot gas .

Hope this helps , if i`ve got it right ? Just remembered that the symbol for ratio of heats is depicted as a curvy- Y .
I see that the slide rule and log tables are still in full use !

All the best with this , regards , Trevor